1. Field of the Invention
The present invention generally relates to a method and apparatus for transmitting data packets in a packet stream over an unreliable channel, and, in particular, to transmitting data packets having compressed headers.
2. Description of the Related Art
Several communication technologies exist for transmitting data from one terminal to another terminal. The most commonly used techniques are cellular telephony and the Internet. Further developments are media-on-demand and conversational services such as Internet telephony. Most of these services require the transport of real-time data including audio and video contents.
The Real-time Transport Protocol (RTP) provides means for such purposes. RTP is an Internet protocol for transmitting data in real-time or nearly real-time. RTP itself does not guarantee real-time delivery of data, but does provide mechanisms for the sending and receiving of applications to support streaming data. Typically, RTP runs on top of the UDP protocol. UDP (User Datagram Protocol) is a connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides no error recovery services, but instead offers a direct way to send and receive datagrams over an IP network.
While RTP has been developed for fixed networks, RTP may nevertheless be used in mobile networks. However, one problem in using RTP over mobile networks is the limited bandwidth in the mobile channel. This is because each of the protocols RTP, UDP and IP has its own header. A packet will then, in addition to link layer framing, have an IP header of 20 bytes, a UDP header of 8 bytes, and an RTP header of 12 bytes, thus summing up to at least 40 bytes.
This header is highly redundant, and to decrease the amount of overhead, header compression mechanisms have been developed. Header compression protocols remove the redundancy of the header and encode the information in an efficient way. This may lead to a compression of the original header down to one byte in the best case.
A system using a header compression protocol is illustrated in FIG. 1. The transmitter comprises a compressor 100 which is used for compressing the original header. The compressed header is then transmitted to the receiver and is there decompressed by the decompressor 110.
The context 120 is the state which the compressor 100 uses to compress the header. The context is a set of variables and consists basically of an uncompressed version of the header fields of the last header. Besides the actual header fields, the context comprises additional variables, such as first order differences of header fields that have been detected to be constant for a series of successive packets. The context can also contain additional information describing the packet stream, for example, the typical inter-packet increase in sequence numbers or timestamps.
In operation, the compressor 100 and the decompressor 110 are required to maintain a common context. When the context 130 of the decompressor 110 is not consistent with the context 120 of the compressor 100, header decompression will fail. This situation can occur when data packets are transmitted over unreliable, e.g. wireless, channels because packets may then be lost or damaged between the compressor 100 and the decompressor 110.
It is therefore necessary to initiate a resynchronization procedure once the context 130 of the decompressor 110 has become invalid. For this purpose, update (UP) packets are provided for transmitting information contained in the context 120 of the compressor 100 to the decompressor 110. Thus, by using UP packets, the context 130 is updated.
The performance of a header compression scheme can be described with two parameters, compression efficiency and robustness. A robust scheme tolerates errors on the link over which header compression takes place without losing additional packets, introducing additional errors or using more bandwidth. Using UP packets increases on the one hand the robustness, but decreases compression efficiency, since UP packets are large in size. Therefore, in addition to UP packet, non-update (NUP) packets are used which are very small and which only depend on the previous UP packet. Thus, NUP packets do not update the context so that, if a NUP packet gets lost, the context 130 of the decompressor 110 continues to be valid, so that the receiver is still able to decompress the following packets.
The packet stream to be compressed usually behaves very regularly. Most of the header fields are constant and do not change during the life-time of the stream. Some fields do change with each packet (e.g. sequence number and timestamp). If the values of these fields are synchronized to the sequence number and thus can be calculated from this number, the stream is regular. Irregularities in these fields disturb this synchronization, e.g. because of a non-linear jump in the RTP-timestamp field. With an irregularity, it is not possible to calculate the values of the changed fields from the sequence number. These irregularities might occur quite frequently, e.g. on the average of every second for a conversational audio stream.
In case an irregular change has occurred, information about the irregular change has to be transmitted to the decompressor. Therefore, either UP or NUP packets have to be extended by this information. This can, for instance, be done by setting an extension bit in the header and by placing the irregularity information into a predefined extension field of the header. However, using extended UP (extUP) packets decreases the robustness, while using extended NUP (extNUP) packets decreases the compression efficiency.